Plasma concentration of low-density lipoproteins (LDL) that transport cholesterol in the human circulation is one of the most important risk factors of cardiovascular morbidity and mortality (1). Excessive amounts of circulating cholesterol are deposited in the walls of coronary vessels causing closure of the vessel lumen and obstruction of blood flow to the heart (and other organs). This disease process is known as atherosclerosis. As a consequence of atherosclerotic events, coronary artery disease and myocardial infarction occur. Given the importance of management of LDL levels in patients, LDL cholesterol remains the primary target of cardiovascular therapy today (2, 3).
Risk for development of diseases and conditions like atherosclerosis, coronary artery disease, and coronary heart disease has been demonstrated to be strongly correlated with high levels of LDL-cholesterol and triglycerides. Elevated levels of low density lipoprotein-cholesterol (LDL-cholesterol) is a significant lipid associated contributor to e.g. coronary heart disease.
Atherosclerosis and its associated coronary artery disease is the leading cause of mortality in the industrialized world. Despite attempts to modify secondary risk factors (smoking, obesity, lack of exercise) and treatment of dyslipidemia with dietary modification and drug therapy, coronary heart disease remains the most common cause of death in the U.S., where cardiovascular disease accounts for 44% of all deaths, with 53% of these associated with atherosclerotic coronary heart disease.
A number of biochemical pathways that affect plasma levels of cholesterol are known and have been considered as targets in therapeutic intervention. These steps include the rate with which cholesterol is produced in the organism and introduced into very low-density lipoproteins (VLDL), the extent of conversion of VLDL into LDL, as well as the efficiency of LDL clearance into hepatic tissues. Furthermore, the conversion of cholesterol into bile acids that are secreted into the gut affect circulating lipid levels.
The Vps10p-Domain Receptor Family
The present inventors have studied the effect of modulation of activity of Vps10p-domain receptors on plasma levels of LDL-cholesterol and triglycerides. The memhers of this family of receptors are Sortilin, SorLA, SorCS1, SorCS2 and SorCS3.
Sortilin
Sortilin, the archetypal member of the Vps10p-domain receptor family is occasionally also referred to as Neurotensin receptor 3 (NTR3), Glycoprotein 95 (Gp95) or 100 kDa NT receptor. Human Sortilin is accessed in Swiss Prot under ID No. Q99523.
Sortilin, (SEQ ID NO. 1) is a type I membrane receptor expressed in a number of tissues, including the brain, spinal cord, testis, liver and skeletal muscle (6-7). Sortilin belongs to a family of receptors comprising Sortilin, SorLA (8), SorCS1, SorCS2 and SorCS3.
All the receptors in this family share the structural feature of an approximately 600-amino acid N-terminal domain with a strong resemblance to each of the two domains which constitute the luminal portion of the yeast sorting receptor Vps10p (9). The Vps10p-domain (Vps10p-D) that among other ligands binds neurotrophic factors and neuropeptides (10-14), constitutes the entire luminal part of Sortilin (sSortilin) and is activated for ligand binding by enzymatic propeptide cleavage (10, 11). Sortilin is a multifunctional type-1 receptor capable of endocytosis as well as intracellular sorting (9-11), and as shown recently, it also engages in signaling by triggering proneurotrophin-induction of p75NTR-mediated neuronal apoptosis (12, 13, 18, 19). Sortilin is synthesized as a proprotein, which is converted to mature Sortilin by enzymatic cleavage and removal of a short N-terminal propeptide. Only the mature receptor binds ligands and interestingly, all its known ligands, e.g. Neurotensin (NT), lipoprotein lipase, the proforms of nerve growth factor-β (proNGF) and brain derived neurotrophic factor (proBDNF), receptor associated protein (RAP), and its own propeptide, compete for binding (11-13, 16), indicating that the diverse ligands target a shared or partially shared binding site. NT is a tridecapeptide, which binds to Sortilin, SorLA and the two G-protein coupled receptors NTR1 and NTR2 (10, 20-22). The physiological role of NT in relation to Sortilin has not been fully elucidated (23), still NT is an important tool, as it inhibits all other ligands from binding to the Sortilin Vps10p-D.
SorLA
Sorting protein-related receptor abbreviated SorLA (Swiss prot ID no Q92673), also known as LR11, is a 250-kDa type-1 membrane protein and the second member identified in the Vps10p-domain receptor family SorLA, like sortilin, whose lumenal domain consists of a Vps10p domain only, is synthesized as a proreceptor that is cleaved by furin in late Golgi compartments. It has been demonstrated that the truncation conditions the Vps10p domain for propeptide inhibitable binding of neuropeptides and the receptor-associated protein. It has been demonstrated (21) that avid binding of the receptor-associated protein, apolipoprotein E, and lipoprotein lipase not inhibited by propeptide occurs to sites located in other lumenal domains. In transfected cells, about 10% of fullength SorLA is expressed on the cell surface capable mediating endocytosis. The major pool of receptors is found in late Golgi compartments, and interaction with newly synthesized ligands has been suggested.
SorCS-3
SorCS1 (Swiss prot ID no Q8WY21), SorCS2 (Swiss prot ID no Q96PQ0) and SorCS3 (Swiss prot ID no Q9UPU3) constitute a subgroup of mutually highly similar proteins containing both a Vps10p-D and a leucine-rich domain bordering the transmembrane domain (14, 26). SorCS1 may play an important role outside the nervous system as its region on the gene was identified as a type 2 diabetes quantitative trait locus in mice (27), and variations in the human SorCS1 gene are associated with diabetes-related traits (28). Further indications in this direction are presented in another study (29) wherein SorCS1 is associated with the major glucose-controlling 16-Mb Niddm1i region in the diabetic GK rat, a region which causes defective insulin secretion and which also corresponds to loci in humans and mice associated with type 2 diabetes.